In a broadband network that supports Internet industry, optical communication is adopted. A laser diode using a compound semiconductor such as Group III-V and Group II-VI is used for optical transmission and reception in the optical communication.
In the meantime, information processing and storage are performed in LSI based upon silicon and the transmission of information is performed by a laser based upon a compound semiconductor. A field of research that tries to realize close optical wiring between silicon chips or in a silicon chip by an optical element using silicon is called silicon photonics. This is technique that tries to produce an optical element using a refined silicon line widely popularized in the world. Currently, a large scale integrated circuit (LSI) based upon a complementary metal-oxide semiconductor (CMOS) is produced in these silicon lines. However, it is considered that in future, merged circuit technology of photonics and electronics acquired by integrating an optical circuit by such silicon photonics with a CMOS circuit will be realized.
In silicon photonics, the most challenging object is a light source. The reason is that as silicon and germanium in bulk are an indirect semiconductor, their efficiency of light emission is extremely bad.
Then, a method of converting silicon and germanium to a direct-transition semiconductor to make the silicon and the germanium efficiently emit is proposed.
For one of methods of converting germanium to a direct-transition semiconductor, a method of applying extensional strain is known. When extensional strain is applied to germanium, energy at a point ┌ in a conduction band decreases according to the magnitude of the strain. When energy at the point ┌ becomes smaller than energy at a point └ as a result of applying extensional strain, germanium converts to a direct-transition semiconductor (refer to Patent Literatures 1 to 6 and Non-patent Literatures 1, 2).
In Non-patent Literature 1, it is reported that germanium converts to a direct-transition semiconductor by applying extensional strain of approximately 2 GPa. Besides, for a method of production, Patent Literature 2 discloses a method of directly epitaxially growing a germanium layer on a silicon wafer and applying extensional strain to the germanium layer utilizing difference in a coefficient of thermal expansion between silicon and germanium. In addition, as an energy gap between a point └ at the bottom of a conduction band of germanium and a point ┌ which is energy of direct transition is 0.136 eV and is small, carriers are also injected at the point ┌ if carriers are injected at high density even if germanium does not completely convert to a direct-transition semiconductor, and an electron and a hole can be recombined in a direct transition type. Patent Literature 3 discloses technique for producing a laser diode by epitaxially growing a germanium layer to which extensional strain of 0.25%, is applied on a silicon substrate, injecting carriers at high density though the germanium layer does not convert to a direct transition type and emitting light. Non-patent Literature 2 discloses a light-emitting diode (hereinafter called LED) produced using a germanium layer epitaxially grown on a silicon substrate. Patent Literature 4 discloses technique for forming a light-emitting element by applying extensional strain to silicon. Moreover, Patent Literature 5 discloses a germanium laser diode using Purcell effect caused by strongly confining light in a germanium layer.
For technique for converting an indirect semiconductor to a direct-transition semiconductor in addition to the method of using extensional strain, a method called valley projection of using nanostructure of silicon is known. Since a region in which electrons spatially move around is limited in silicon in nanostructure, the momentum of the electron effectively decreases. In a substance such as silicon and germanium, a direction in which an electron has momentum depends upon proper band structure. The valley projection means a method of confining electrons in nanostructure in a direction in which the electron has momentum. As a result, the momentum of the electron is effectively turned zero. That is, the valley projection is a method of effectively turning a valley of energy in a conduction band a point ┌ and falsely converting to a state of direct transition. For example, since the bottom of a conduction band exists in the vicinity of a point X in band structure in bulk of silicon, a valley of energy can be effectively turned a point ┌ by making a (100) plane its surface and thinning the silicon and the silicon can be falsely converted to a direct-transition semiconductor. Besides, in the case of germanium, since the bottom of a conduction band is located at a point └ in bulk, a valley of energy can be effectively turned a point ┌ by forming a thin film having a (111) plane on its surface and the germanium can be falsely converted to a direct-transition semiconductor. Patent Literature 1 discloses an element that makes extremely thin monocrystalline silicon efficiently emit by directly connecting the extremely thin monocrystalline silicon having a (100) plane on its surface to an electrode and injecting carriers in a horizontal direction to a substrate.
In addition, Patent Literature 6 discloses double heterostructure of silicon and germanium that silicon is thinly laminated on germanium and current is made to flow in a direction of the thickness of the extremely thin silicon.